1. Field of the Invention
The present invention relates to a catadioptric lens system which forms an image by means of a plurality of light reflecting and refracting surfaces. The catadioptric lens system is basically constructed as a form of reflecting telescope and is used as an exchangeable objective lens for a camera.
2. Description of the Prior Art
It is well known that the catadioptric lens system is almost incapable of being provided with a conventional iris diaphragm for controlling the light transmission through the lens system by means of stopping-down the iris diaphragm. This is because of the complex light path in the system, bulkiness of the construction and the problem of cost.
The conventional catadioptric lens system, accordingly, adopts a plurality of ND (neutral density) filters of various light transmission, one of the ND filters being exchangeably inserted in the lens system at the image side end portion thereof to control the light transmission through the lens system by means of attenuating light passing through the ND filter. The disadvantage of such a conventional catadioptric lens system is that a great number of various ND filters have to be prepared if gradual steps of change in light transmission are desired. Additionally, it is troublesome for a user to select one of the great number of ND filters and exchange the one for another. On the contrary, a limited number of available ND filters can only realize a sudden change in light transmission per one step of exchange of ND filters.
U.S. Pat. No. 4,054,890 is cited in that a general light control by means of electrochromic device is disclosed. The electrochromic device, however, is used in the patent to form a shutter or an iris diaphragm type aperture stop for a lens system of a conventional camera lens system.